Fibrous elements that work effectively in demisting, column packing and filtration applications ideally should possess at least some of the characteristics of being inert in the system of use, have a high temperature stability, a low pressure drop, not wetted and/or swollen by the fluid system in use, have good flame arresting properties in the presence of flammable fluids, have sufficient conductivity to render anti-static or grounding properties, can be used with an electrical field or gradient electrical field to enhance separation and have good vibration stability without vitrification with time at temperature. No single material or combination of materials prior to the present invention is capable of being customized to contain a desired combination of these properties.
Mist eliminator mesh pads are typically pads composed of elements, such as knitted wire mesh, and are commonly placed in a gas-liquid contact apparatus to remove mist from a mist-containing gas stream. Typically, such mist eliminator mesh pads are composed of fibrous or filament elements, such as four to fifteen mil diameter stainless steel wire, are arranged from about three to twenty-four inches in thickness, have a density ranging from about four to fifteen pounds per cubic foot and range in diameter from about one to thirty feet depending upon the gas-liquid contact apparatus in which the pads are employed Such mist eliminator mesh pads are generally effective in removing droplets as small as one to five micrometers from mist containing gas streams.
The capacity of a mist eliminator mesh pad in a gas-liquid contact apparatus, i.e. the maximum gas velocity of the gas stream through the mesh pad, is generally limited by the mesh pad's ability to drain rapidly the coalesced liquid collected by the mesh pad. One attempt to increase the capacity of mist eliminator mesh pads and to reduce the mesh pad's pressure drop has been the employment of drainage cylinders or ancillary rolls of wire mesh fixed to the bottom of conventional mist eliminator mesh pads. Such drainage cylinder of ancillary rolls is provide for localized, separate regions of flow interruption and interception, thereby creating a preferential drainage foci. (See for example U.S. Pat. No. 4,022,593, issued May 10, 1977, hereby incorporated by reference in its entirety.)
In some limited cases, it has been the past practice to employ variable high and low density mesh pads in a vapor phase intercept pattern to enhance mist elimination performance. In such cases, the lower portion of the mesh pad is formed of a low density material to promote rapid and easy draining of coalesced liquid and to aid in working away precipitated material from the pad, while the upper portion of the pad is formed of a high density material to collect liquid particulates from the upwardly flowing, mist containing vapor stream.
It is desirable to provide an improved mist eliminator mesh pad in order to improve the mesh pad capacity and to provide for reductions in pressure drop compared to conventional mesh pads.
There have been many recent advances in the use of pulsed field electrophoresis and the separation of molecules based on their migration through an electrical field. Electrophoresis separation is generally accomplished by establishing an electrical field between two electrodes in a gel such as an argose gel. Column separation of molecules has been accomplished using electrically conductive polymers such as polyethylene oxides or polypyrrole copolymers. However, such polymers and gels have only found limited application and cannot be utilized in many common solvent systems. Also, the prior conductive polymers do not provide a sufficient variant in pulsed fields to perform many simple separations.
There is a need to provide a means for separating molecules in solution, for example, removal of by products in chemical reactions, desalination, removal of solvents, and the like.
U.S. Pat. No. 4,837,076 to McCullough et al, which is herewith incorporated by reference discloses a class of carbonaceous fibers which may be used in the present invention.
U.S. Pat. No. 4,744,806 to Ozolins et al, which is herewith incorporated by reference, discloses demister pads and apparatus which are similar to the apparatuses and pads of the invention except that the pads are metallic and cannot be used with an electrical field for separation.
The carbonaceous fibers of the invention according to the test method of ASTM D 2863-77 have an LOI value greater than 40. The test method is also known as "oxygen index" or "limited oxygen index" (LOI). With this procedure the concentration of oxygen in O.sub.2 /N.sub.2 mixtures is determined at which a vertically mounted specimen is ignited at its upper end and just continues to burn. The size of the specimen is 0.65.times.0.3 cm with a length from 7 to 15 cm. The LOI value is calculated according to the equation: ##EQU1##
The LOI values of different materials are as follows:
______________________________________ polypropylene 17.4 polyethylene 17.4 polystyrene 1.1 rayon 18.6 cotton 20.1 nylon 20.0 polycarbonate 22 rigid polyvinyl chloride 40 stabilized polyacrylonitrile 40 graphite 55 ______________________________________
The term "non-graphitic" as used herein relates to those carbonaceous fibers having an elemental carbon content of less than 98%, preferably, less than 92%, as further defined in U.S. Pat. No. 4,005,183, which is herein incorporated by reference.
The term "carbonaceous fibers" refers to fibers having a carbon content of at least 65% after an irreversible chemical change brought about by heat treatment as disclosed in U.S. Pat. No. 4,837,076.